U.S. Pat. No. 4,303,472 (the disclosure of which is incorporated herein by reference) describes and claims a method for forming a tubular filter element which includes the steps of:
(a) forming a slurry of fibers in a liquid;
(b) introducing the slurry under pressure into the top of an annular molding space defined between a central core, a vertical cylindrical screen spaced from and outward of said core and a support defining a lower boundary for the molding space so that a mass of fibers becomes compacted on the screen and liquid is discharged from the molding space through the screen;
(c) progressively increasing the height of the effective open area of the cylindrical screen by moving upwardly a sleeve in sliding contact with the cylindrical screen at a rate substantially equal to the rate at which the height of the mass of fibers increases above the support; and
(d) removing the resulting tubular mass of fibers from the molding space.
In a practical embodiment, the filter element comprises a mass of borosilicate glass microfibres bounded by a foraminous outer support sheet or by foraminous inner and outer support sheets, e.g. of steel mesh with an open area of 45-70%. The borosilicate fibers are dispersed in water in a blending tank under mechanical agitation, and an acid, e.g. hydrochloric or sulfuric acid is added to give a pH of 2.9-3.1 at which the dispersion is stable, the fiber to water ratio being 0.01-0.5 wt %, typically 0.05 wt %. The resulting slurry is introduced into the molding space under a pressure of typically 290-480 Pa (6-10 p.s.i) and molded as described above. The sleeve is raised progressively at substantially the same rate as that at which the height of the fiber mass increases in order to maintain a flow of the dispersion to the point where the mass of fibers is building up, after which air may be passed through the molded element to reduce the content of residual water. The formed filter element is removed from the molding space, oven dried, resin impregnated and heated to harden the resin. As originally disclosed, the resin could be e.g. a silicone or an epoxy resin and could be impregnated in a solvent such as acetone, but it is now preferred that the resin should be a phenolic resin which may be impregnated as an aqueous solution. The fibers in a finished filter element produced by the above method are predominantly layered in planes perpendicular to the direction in which the dispersion flows into the molding space, and the same packing pattern arises throughout the range of forming pressures that can be used in practice. This non-random packing pattern results in a filter element that provides efficient depth filtration and has an advantageous combination of properties including high burst strength and low pressure drop. The molded tubular elements may be bonded to end caps to complete the formation of the filter.
The above process has been used e.g. to manufacture air/oil separators designed to remove water and oil mist particles generated in screw or sliding vane compressors or in vacuum pumps where the size of the particles generated lies in the range 0.3-1.5 microns (μm) and also to manufacture in-line filters for removing oil, water and contaminants from a stream of compressed air. Filters for the above purposes are described in our U.S. Pat. No. 5,129,923 the disclosure of which is also incorporated herein by reference.
Our EP-A-1385598 (the disclosure of which is incorporated herein by reference) discloses that improved consistency in molding fibrous filter elements from slurry can be achieved by providing a slurry holding tank or tanks separate from a slurry mixing tank or tanks, recycling aqueous liquid from the or each molding rigs to a reservoir also fed with mains water, and controlling the total volume of water in the apparatus so that mains water need be added to the reservoir only in relatively small increments compared to the volume of aqueous liquid already in the reservoir which then contains predominantly recycled aqueous liquid of known composition or contains liquid of gradually changing composition. By having liquid in the reservoir tank whose composition is largely known or gradually changing, abrupt changes in composition of the liquid in the mixing tank or tanks is avoided and instability and over-shoot in systems for controlling the composition of liquid in the mixing tank or tanks can be avoided Furthermore mixing can be continued until it has been determined that a freshly made slurry mix is of the correct consistency and free of fiber aggregates before the slurry is allowed to enter the holding tank or tanks and to be supplied to the molding rig or rigs.